To meet the demand for wireless data traffic having increased since deployment of 4G (4th-Generation) communication systems, efforts have been made to develop an improved 5G (5th-Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (COMP), reception-end interference cancellation, and the like.
In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
Currently, a series of studies to operate a long term evolution (LTE) system in an unlicensed frequency band have been conducted. The LTE system operating in the unlicensed frequency band is referred to as an LTE-unlicensed (LTE-U) or unlicensed LTE system.
The unlicensed frequency band is not a frequency band that is allocated only for a specific system. Therefore, if an arbitrary communication system is allowed to use an unlicensed frequency band by a regulatory agency, such as Federal Communications Commission (FCC), and complies with the rules established by the regulatory agency, the communication system may perform communication by transmitting and receiving wireless signals through the unlicensed frequency band. Wi-Fi or Bluetooth, which is currently used by many people, corresponds to the typical technology using the unlicensed frequency band.
Currently, with the significant increase in mobile data traffic through LTE or LTE-advanced (LTE-A), carriers or communication service providers have shown a great interest in securing the frequency band capable of accommodating the increasing mobile data traffic. The most basic way to do this is to purchase a licensed frequency band that is allocated for the LTE system. However, since frequency resources are public resources that are strictly managed and controlled by the government, purchasing the licensed frequency band requires a lot of time and cost, and the complicated procedures. Therefore, some carriers and communication chip makers have shown their intentions to operate the LTE system in the unlicensed frequency band near 5 GHz. It is known that in the 5 GHz band, the LTE system can utilize a wide frequency band of about 500 MHz. Therefore, if the unlicensed frequency band of 5 GHz is effectively utilized, it is expected to significantly increase the capacity of the LTE system.
However, the unlicensed frequency band near 5 GHz may be used not only by the LTE system but also by other communication systems (typically, the Wi-Fi system). Therefore, the consideration for allowing the LTE system not to damage other communication systems while complying with all the regulations that the LTE system should comply with in the unlicensed frequency band should be reflected in the LTE-U system.
If the LTE system operates in the unlicensed frequency band, the performance of the Wi-Fi system that has been using the unlicensed frequency band cannot but be deteriorated. This is because despite the limited frequency bandwidth, the number of wireless communication devices desiring to use the unlicensed frequency band increases. However, if the LTE-U system causes additional performance degradation of the Wi-Fi system in addition to the performance degradation due to the increase in the number of communication devices, this may be a factor to hinder the introduction of the LTE-U system. Therefore, the LTE-U system should be designed considering not only its own performance but also the performance of other wireless communication devices that use the same frequency band, such as the Wi-Fi system.
FIG. 1 illustrates a carrier sense multiple access/collision avoidance (CSMA/CA) basic operation in a media access control (MAC) protocol of a Wi-Fi system according to the related art.
Referring to FIG. 1, the basic MAC protocol and performance degradation factors of Wi-Fi will be described. The MAC protocol of Wi-Fi typically uses CSMA/CA.
If a wireless local area network (WLAN) transmitter 1 (WLAN TX 1) transmits data 100 to a WLAN receiver 1 (WLAN RX 1) using a specific channel, the RX 1 may transmit an acknowledgement (ACK) 104 to the TX 1 in the channel after a time of short interframe space (SIFS) 102.
At this point, if a nearby TX (e.g., WLAN TX 2) senses (or detects) the channel and determines that the channel is in a busy state, the nearby TX may not transmit data, deferring an access to the channel as shown by reference numeral 106. On the other hand, as a result of the sensing (or detection), if the TX 2 determines that the channel is in an idle state, the TX 2 may start backoff 110 after a time of distributed coordination function (DCF) interframe space (DIFS) 108, recognizing that the data transmission of the TX 1 is terminated. The backoff is an operation in which a transmitter chooses a backoff number having a value within a certain range and waits for a time corresponding to the chosen backoff number. In other words, a transmitter that has chosen the smallest backoff number through the backoff operation may first perform transmission.
The TX 2 that has chosen the smallest backoff number in the backoff process may transmit data 112 over the channel, and other nearby TX may wait without transmitting data, determining that the channel is in the busy state.
The backoff number is determined as an arbitrary integer between 1 and a contention window (CW), and a binary exponential backoff algorithm may be used in which a value of the CW is doubled each time data transmission is failed due to occurrence of collision.
When CSMA/CA is used in Wi-Fi, performance degradation may occur due to the following factors.
In a first case, after sensing an idle channel, a plurality of TXs choose the same backoff number in the backoff process, and perform transmission at the same time. In this case, the signals transmitted from the plurality of TXs may interfere with each other, making it difficult to successfully transmit and receive the signals.
In a second case, although a transmitting terminal has performed transmission as the transmitting terminal determines that the channel is in the idle state, when performing channel sensing (or channel detection), the channel may be in the busy state, for a receiving terminal. This case may mainly occur when a hidden node in view of a TX (i.e., another TX out of the sensing area (or detection area) of the TX) is performing transmission. The second factor is generally referred to as a hidden node issue.
In Wi-Fi, the TX and the RX may address the hidden node issue by using a request to send (RTS) and a clear to send (CTS), respectively.
FIG. 2 illustrates a hidden node issue of a Wi-Fi system according to the related art.
Referring to FIG. 2, when a TX A 200 is transmitting data to an RX 1 202 over a specific channel, a nearby TX B 204 may not determine whether the TX A 200 out of its own sensing area is presently performing transmission. In other words, the TX B 204 may fail in sensing the state of the specific channel as a busy state as shown by reference numeral 208. If the TX B 204 senses, as an idle state, the state of the channel that the TX A 200 is transmitting, and transmits data to the RX 1 202 in the channel as shown by reference numeral 206, the RX 1 202 may receive signals from both of the TX A 200 and the TX B 204. Therefore, the signals from the TX A 200 and the TX B 204 may act as interference to each other, making it difficult to successfully transmit and receive the signals.
FIG. 3 illustrates a solution to a hidden node issue through RTS and CTS in a Wi-Fi system according to the related art.
Referring to FIG. 3, the TX A 200 may inform its nearby nodes (TXs or RXs) that the TX A 200 will transmit data to the RX 1 202, by transmitting RTS 300 before transmitting the data. Further, upon receiving the RTS 300, the RX 1 202 may inform its nearby nodes that the RX 1 202 will receive data from the TX A 200 in the future, by transmitting CTS 302. Therefore, the nearby nodes that have receive the RTS 300 or the CTS 302 may not perform data transmission until the data transmission/reception between the TX A 200 and the RX 1 202 is terminated. By receiving the CTS 302, the TX B 204 may also stop or delay the transmission. In this way, the Wi-Fi system may address the hidden node issue by using the RTS or CTS.
However, the LTE-U system shows another type of the hidden node issue, which is different from that of the Wi-Fi system.
FIG. 4 illustrates a transmission frame structure for a description of a transmission operation by channel sensing in an LTE-U system implementing carrier aggregation (CA) using an unlicensed frequency band according to the related art.
Referring to FIG. 4, in the LTE-U system implementing CA, a licensed band (or a licensed frequency band) may be used as a primary cell (PCell) or a primary carrier 400, and an unlicensed band (or an unlicensed frequency band) may be used as a secondary cell (SCell) or a secondary carrier 410.
An evolved Node B (eNB) may schedule user equipment (UE) transmission in the SCell 410 through the PCell 400. In other words, the UE may be allocated uplink transmission resources in the SCell 410 through the PCell 400.
In the unlicensed band (such as the SCell 410), a TX may sense a channel, and transmit data, if the TX determines a state of the channel as an idle state. For example, before transmitting downlink data 414 to the UE over a channel in the unlicensed band 410, the eNB may sense the channel as shown by reference numeral 412 to determine whether the channel is in the idle state. Further, the UE may sense the channel as shown by reference numeral 416 before transmitting uplink data 418 to the eNB over a channel in the unlicensed band 410.
Generally, the UE is allocated uplink transmission resources through the PCell 400, k subframes before the transmission time. If the UE senses that the allocated transmission resources are used for transmission 420 of a Wi-Fi device, the UE may not transmit uplink data even though the UE has reached the allocated uplink transmission period.
In the LTE-U system, in the case of downlink transmission, a TX is one eNB, but RX may be a plurality of UEs. The characteristics that a plurality of UEs exist may act as a cause of the occurrence of the hidden node issue, which does not exist in the Wi-Fi system, for the LTE-U system. In the Wi-Fi system, basically, one TX transmits data to one RX at a specific time. However, in a downlink of LTE-U, generally, one TX (i.e., an eNB) may transmit data to a plurality of RXs (i.e., UEs) at a specific time. Therefore, in LTE-U, the sensing results (or detection results) of TX and RX may be classified as shown in Table 1 below.
TABLE 1Channel sensingChannel sensingCaseresult of eNBresult of UERemarks1IdleAll UEs are idleObserved in Wi-Fi2IdleAll UEs are busyObserved in Wi-Fi3IdleSome UEs are idleNewly occur in LTE-USome UEs are busy4BusyAll UEs are idleObserved in Wi-Fi5BusyAll UEs are busyObserved in Wi-Fi6BusySome UEs are idleNewly occur in LTE-USome UEs are busy
In Case 6 shown in Table 1, it is a situation that newly occurs in LTE-U. However, since an eNB which is a TX has sensed the channel state as a busy state, downlink transmission is not performed (so the hidden node issue may not occur). Therefore, in Case 6, there is no need to define additional operations of the eNB.
On the other hand, in Case 3 shown in Table 1, since the eNB has sensed the channel state as an idle state, the condition that the eNB can transmit data, has been met. However, since some UEs among the UEs scheduled by the eNB have sensed the channel state as an idle state and some other UEs have sensed the channel state as a busy state, some UEs that have sensed the channel state as a busy state may have failed to meet the condition that the UEs can receive data. In other words, since some UEs that have sensed the channel state as a busy state may be communicating with a hidden node, the UEs may fail to correctly receive a downlink signal from the eNB.
Therefore, it is necessary to define how the eNB should operate to address the issue that may occur in the LTE-U system.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.